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Robot Mechanics

25

Robot mechanics

Materials 26Materials 26Materials 26Materials 26

Fixings 29Fixings 29Fixings 29Fixings 29

Tools 31Tools 31Tools 31Tools 31

Planning a mobile robot body 36Planning a mobile robot body 36Planning a mobile robot body 36Planning a mobile robot body 36

Wheels 39Wheels 39Wheels 39Wheels 39

Motors 46Motors 46Motors 46Motors 46

Solenoids 50Solenoids 50Solenoids 50Solenoids 50

Suppliers 51Suppliers 51Suppliers 51Suppliers 51

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The Robot Builder’s Cookbook

26

Materials

The materials for building the body or framework of the robot must be strong enough for

the job, easy to work, durable and low cost. Also it should look good — have a shiny or

attractively coloured surface.

Some kinds of plastic food container have all of these qualities. Project 1 illustrates how

to build the robotic mechanisms and circuits into a ready-made box. If there happens to

be a spare unused box in the kitchen cupbooard, it costs nothing. The main snag is that it

may not be exactly the right shape or size.

Converting a sandwich box into a robot is a short-cut way of getting into robotics, and the

programs it runs can be really high-level, but a purpose-built body is more professional.

The following sections describe some of the most popular materials.

Aluminium stockAluminium stockAluminium stockAluminium stock

Most DIY stores hold a range of aluminium stock, and it is inexpensive. It is usually sold

in lengths of two metres and there is a variety of sizes and cross-sections. The drawing

shows some of them.

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Aluminium is also available in sheets, commonly 1.5 mm thick.

This material is easy to drill and to cut, using a hack-saw. Strip and square-sectioned

stock can be bent by hand, provided it is not too thick. So can rod.

It seems too obvious to point out that aluminium has the advantage of being a low-

density metal. Lightweight yet rigid frames mean that low power motors can be used to

move them. This in turn means that low-power batteries are needed to drive them.

Project 3, the Gantry, is an example of an aluminium framework. This was built using

only two kinds of stock, strip and channelling.

Brass stockBrass stockBrass stockBrass stock

This is useful for some of the smaller parts of mechanisms. It is obtainable from model-

making stores. Brass is available in most of the same sections as aluminium stock, but

smaller dimensions. It is often sold in 200 mm lengths. Brass is more expensive than

aluminium but fortunately we do not need a lot of it.

Brass is easily worked with drill and hacksaw. The thinner stocks can be bent by hand.

The photo of the gripper on p. 000 show how it can be bent to form jaws.

Its distinguishing feature is that, it is reasonably rigid but has a degree of springyness

that aluminium does not have. This is why it or similar alloys are used for electrical

contacts, and various kinds of spring clip.

PlasticPlasticPlasticPlastic

Model shops stock a wide range of plastic rod, tubing, angle, channeling and sheets.

These are are in small sizes, being intended for scale models, but can be useful. Usually

they are high impact polystyrene and special adhesive is used when building up boxes

and frames.

Another source of plastic parts is the DIY store. The plumbing department stocks a range

of tubing and othe plumbing parts that can be used in robot building.

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Examples of plumbing parts are the plastic pipe caps used as light shields for the IR

sensors of the Quester (p. 000).

The gardening department of the DIY store may provide handy plastic tubing used in

garden reticulation systems. The spacers that separate the decks of the Quester are cut

from long PVC riser tubing.

One of the more generally useful materials for robot construction is 3 mm expanded PVC

board. It is often used by signwriters and a visit to a local signwriter may provide some

offcuts. If this fails, try a local plastics company. The board comes in sheets about 1 m by

2 m.

Unlike expanded polystyrene, which is soft and crumbly, expanded PVC is firm. Yet is

has a certain amount of compressibility which means that nuts and bolt-heads sink a

fraction of a millimetre into the surface when tightened. This makes them less likely to be

loosened by vibration. The sheet is easy to drill and cut. A steel rule and sharp craft knife

are all that is needed for cutting straight-edged pieces.

The sheet is manufactured in a range of attractve colours, The Quester, for example, is

bright tomato-red.

Foam board is a similar material. It consists of a 5mm thick sheet of solid plastic foam

coated on both sides with a plastic film. It is white on one side and coloured on the other.

The board is not quite as strong as expanded PVC, but is just as easily worked and

suitable for small light-weight robots, such as the Scooter and Android. Robot bodywork

and other structures can be assembled by using craft glue, as explained on pp. 213-215.

There is plenty of scope for givng the robot a really unique appearance.

Foam board is sold at office materials stores. The brand (Elmers) used for the Android is

supplied in sheets that measure 568 mm × 762 mm, which is a convenient size.

WoodWoodWoodWood

Wood is rarely thought of as a robot-building material but, at times, it can be just what

we need. It is strong for its weight and easily cut, drilled, painted, carved and glued.

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As well as a wide range of building and joinery timbers, mostly too large for robot

making, DIY and model-maker’s stores sell a very easily workable timber known as

Balsa. Because of its low density and easy workability it is a firm favorite with flying

model aeroplane constructors. For the same reasons it is useful for robot building too.

The Android shows one way of using it.

Fixings

These hold the parts of the robot together — mostly nuts and bolts.

Buy in a stock of nuts and bolts in the sizes most suitable for small structures. The most

generally useful size of bolts is M3 (3 mm diameter) and you need nuts to fit. Occasioally

a smaller size is required. For instance, a motor may have mounting holes with M2.5 or

M2 threads. Small parts such as microswitches may have 2 mm unthreaded mounting

holes.

You need an assortment of the different lengths. The 10 mm and 15 mm size cover most

needs, but sometimes longer bolts such as 25 mm are wanted, and a few of the 6 mm size.

Washers have several different functions. Plain washers, placed next to the head of the

bolt, help to spread the load at that point. They are useful when bolting a relatively

massive item, such as a motor, to a relatively flexible panel. Shake-proof and spring

washers help prevent the nuts from loosening. Use them for bolting metal parts to other

metal parts. They are not needed when bolting to expanded PVC sheet and the material

itself is suitably springy.

Nylon nuts and bolts, from electronic parts suppliers, are necessary if there is a risk of

the bolts causing short-circuit. This could happen if a circuit-board is bolted to a metal

panel. In such cases use nylon bolts and nuts or plastic stand-offs.

Spacers are short tubes, length 6 mm to 38 mm, made of metal or nylon. They are

intended for holding a circuit board clear of the panel on which it is mounted but have

several other uses. We sometimes refer to the shorter ones as collars.

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There are dozens of kinds of adhesive, of which we employ just three. For routine fixing,

general adhesives such as UHU®,, Bostick®, or similar products are our standby.

Another general glue, which sticks expanded PVC sheet and Foam Board is a variety of

craft glue called Sticky Craft Glue, made by CraftSmart. It is milky when applied but dries

clear. Clamp the pieces under slight pressure while the glue sets. Examine it from time to

time at first to check that the pieces have not slipped.

Super glues are quick setting and strong. We use a variety of this known as Fix-Lock

anaerobic adhesive. A drop applied to a nut and bolt runs into the narrow space between

them and sets hard. This locks the nut on to the bolt, preventing it from working loose. A

locking adhesive such as this is invaluable when building robots from metals parts.

Although it holds the nut secure, a little force with a spanner will loosen it iff necessary.

It should really be classed as a tool but it seems more sensible to describe it along with the

adhesives. The tool is the glue gun, which melts glue sticks and has a nozzle for applying

the molten glue to the workpiece. A glue gun is a handy tool to have on the workbench

for all kinds of gluing jobs.

Velcro would seem to have little to do with robots but in fact it can be very good at fixing

things that can not be fixed by nuts, bolts or adhesives. Velcro Sticky Back tape consists of

the usual ‘hook’ and ‘eye’ tapes with strongly self-adhesive backs. Typical AAA and AA

battery holders have no mounnting holes, and there is nowhere they can be drilled to

take a bolt. We use this tape for fixing battery holders and similar items.

Last but by no means least, be sure to have a packof Blu-Tack to hand, as well as a packet

or roll of double-sided self-adhesive tape.

The glue sticks are melted by theThe glue sticks are melted by theThe glue sticks are melted by theThe glue sticks are melted by the

electric heating coil in the gun. Presselectric heating coil in the gun. Presselectric heating coil in the gun. Presselectric heating coil in the gun. Press

the trigger to extrude molten gluethe trigger to extrude molten gluethe trigger to extrude molten gluethe trigger to extrude molten glue

from the nozzle.from the nozzle.from the nozzle.from the nozzle.

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Robot Mechanics

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The tools you need for constructing robots partly depend on the materials you use. For

Foam Board the main tools are a steel ruler, a craft knife and a plastic chopping board

(use one discarded from the kitchen) or cutting mat. You need a few other tools for

mounting the motor and circuit boards. For building an aluminium framed robot such as

the Gantry, a drill press is almost essential and so is a hacksaw. When you have decided

what materials are to be used, select your tools from those described below.

Cutting toolsCutting toolsCutting toolsCutting tools

A junior hacksaw, with a 150 mm long blade is good enough for most jobs, such as

cutting wood or plastic, and for circuit boards. For cutting aluminium or brass stock a

regular hacksaw is faster and gives a straighter cut. If you have problems with cutting

things square or if you need to cut at a particular angle, a mitre saw is a great help. It

keeps the saw plade vertical and perpendicular to the length of the work piece. It has

gauges to help cut pieces to equal lengths. The frame that carries the blade can be rotated

to cut at angles other than 90°, the angle being settable on a graduated scale. A mitre saw

is almost essential for building the Gantry.

Tools

A mitre saw helps keep everything ‘square’.A mitre saw helps keep everything ‘square’.A mitre saw helps keep everything ‘square’.A mitre saw helps keep everything ‘square’.

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Use a medium-sized fine flat file for smoothing off cut edges. A set of needle files is

useful for enlarging holes and shaping small parts. Use a file saw for shaping larger

holes, and many other tasks. The blade is a coarse round file about 3 mm diameter and

175 mm long. It is mounted in a handles. The file saw cuts quickly and is suitable for

cutting metal, wood or plastic.

A reamer can enlarge circular holes up to 18 mm in diameter. It is not an essential tool,

but does the job neatly. While on the subject of cutting large holes, consider getting a

circular hole-saw that attaches to an electric drill. It is supplied with a range of inter-

changeable blades in diameters from 25 mm to 53 mm. Again, this is not an essential tool,

but is the best way of cutting large holes quickly.

The saw blades cut holes of aThe saw blades cut holes of aThe saw blades cut holes of aThe saw blades cut holes of a

range of sizes, centred on therange of sizes, centred on therange of sizes, centred on therange of sizes, centred on the

hole first drilled by the bit.hole first drilled by the bit.hole first drilled by the bit.hole first drilled by the bit.

A file-saw, aA file-saw, aA file-saw, aA file-saw, a

reamer and areamer and areamer and areamer and a

junior hacksaw.junior hacksaw.junior hacksaw.junior hacksaw.

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DrillsDrillsDrillsDrills

Although a hand-turned drill is adequate in many ways, an electric drill is a boon when

there is much drilling to be done. Robot building seems to require a lot of it. If you

already have a small power drill for jobs about the house, it may not be worth while to

get anything more professional. A drill press is not expensive, is so much easier to use

and produces better results.

A drill press helps put the holes in the right places and at right angles.Use itA drill press helps put the holes in the right places and at right angles.Use itA drill press helps put the holes in the right places and at right angles.Use itA drill press helps put the holes in the right places and at right angles.Use it

for aluminium, brass, wood or plastic, preferably running it at its lowestfor aluminium, brass, wood or plastic, preferably running it at its lowestfor aluminium, brass, wood or plastic, preferably running it at its lowestfor aluminium, brass, wood or plastic, preferably running it at its lowest

speed.speed.speed.speed.

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The drill press in the photo has a drive belt transmission for varying the speed. For

drilling plastic it is best to run it at its slowest speed.

There is a safety guard to prevent flying pieces of metal from damaging your eyes, but

we have yet to see any flying pieces. However, wearing a pair of safety goggles is a

sensible precaution, especially when drilling into metal. A pair of heavy-duty gloves are

another safety measure.

Plastic safety goggles protect the eyes when drilling.Plastic safety goggles protect the eyes when drilling.Plastic safety goggles protect the eyes when drilling.Plastic safety goggles protect the eyes when drilling.

A selection of small clamps is handy for holding the workpiece while drilling. You also

need a set of drill bits, with diameters ranging from 1 mm to 6 mm. A centre-punch and

hammer are used for punching the spot where the hole is to be drilled. If the workpiece is

not punched beforehand there is a tendency for the bit to skid away and start the hole in

the wrong place.

A small hobby drill, driven by a low voltage motor is needed for the finer work. It can be

powered from the low-voltage suppy of the electronics bench. Most work on a wide

range of DC voltages from about 6 V to 15 V. Kits of tools for these drills include drill

bits, mini reamers, wire brushes, and felt polishing discs. It may even have a mini circular

saw. The most essential items are the bits, ranging in diameter from 0.8 mm up to 2 mm,

and the reamers.

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Gripping toolsGripping toolsGripping toolsGripping tools

In this category come a pair of standard bull nose pliers and a pair of long nose pliers. It

also includes two sizes of adjustable spanners, with jaws opening up to 12 mm and

36 mm.

There are many kinds of tool for picking up and holding small items, and the most

generally useful is a pair of forceps or tweezers. Get a fairly robust pair (tines about 2 mm

wide) and use them for handling small nuts and bolts, bending wire, levering ICs out of

sockets and dozens of other tasks. Our forceps are used more than any other tool.

On the larger scale a small bench vice holds items being sawn or drilled with the mini

drill. The low-cost vices from hobby stores are made from plastic and have a suction base

for temporarily attaching them to the work bench.

This miniature electric drill isThis miniature electric drill isThis miniature electric drill isThis miniature electric drill is

supplied with a range ofsupplied with a range ofsupplied with a range ofsupplied with a range of

tools.tools.tools.tools.

The magnifier/gripper tool shown in theThe magnifier/gripper tool shown in theThe magnifier/gripper tool shown in theThe magnifier/gripper tool shown in the

photo is another handy item, particularly forphoto is another handy item, particularly forphoto is another handy item, particularly forphoto is another handy item, particularly for

holding small circuit boards and holding small circuit boards and holding small circuit boards and holding small circuit boards and compo-compo-compo-compo-

nents such as plugs and switches whennents such as plugs and switches whennents such as plugs and switches whennents such as plugs and switches when

soldering.soldering.soldering.soldering.

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Other toolsOther toolsOther toolsOther tools

Screwdrivers are obviously essential but only small sizes with regular blades up to 4 mm

wide. Metric bolts usually need a small-size Posidrive screwdriver. A set of jeweller’s

screwdrivers is useful on occasions.

An assortment of small files of various shapes is handy for smoothing cut edges and

shaping parts of mechanisms. A set of needle files is also worth having.

An engineer’s steel rule 300 mm long, graduated in millimetres, completes the tool kit.

Planning a mobile robot body

Things to think about before starting on the building.

• Chassis:

◊ Ready-made chassis such as a plastic food storage box. Easy and quickly

built. See Project 1, the Scooter, p. 165.

◊ An existing toy. Genreally easy and quick, but there could be snags. See

Project 3, p. 246.

◊ Chassis made from sheet plastic or foam board. Easy but takes longer. Gives

scope for invention. See Project 2, the Android, p. 209 and Project 4, p. 258.

◊ Chassis made from sheet aluminium. You need tools and experience to make

it.

• Wheels:

◊ How many? Three is a popular number.

◊ Drive (traction) wheels at front or rear.

◊ Steering by tank method (two independent drive wheels, two motors).

◊ Steering by automobile method, one motor, with differential gear

(complicated)

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Copyright © 2007, Thomas Murray

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• The terrain it is to run on:

◊ Hard, level, smooth surface such as wooden or tiled floor, or concrete or

brick paving. Wheels may need composition treaded tyres for good grip.

◊ Carpeted floor. Mount wheels low down on the chassis to give clearance

between the chassis and the carpet. Rugs are often difficult to negotiate,

especially rugs with fringes. Wheels of larger diameter ride on to rugs more

easily. Steps and stairs are really difficult — instead of wheels use tracks or

abandon wheels and go for legs.

◊ Lawn. Large diameter wheels, with good clearance.

◊ Sloping terrain. Make the robot broad and squat, so that it does not

overbalance.

• Size:

◊ Small size (under 200 mm in all directions) is good for robots run at home.

They can find their way between the furniture more easily.

◊ Small size may not provide enough space for battery, motors, all the sensors

and actuators that are intended.

◊ Small size may lead to cramped conditions, making it hard to access circuits

for testing, and difficult to insert and remove the PIC microcontroller.

◊ Large size leads to greater weight, need for stonger chassis, need for more

power to drive it, more powerful (larger and heavier) motors, more powerful

(larger and heavier) battery — the situation can get out of hand! Small is

beautiful!

• Shape:

◊ A robot that is about as long as it is wide turn much more easily in a confined

space (such as the average family room).

◊ A robot that is much taller than it is wide in more likely to fall over on

uneven surfaces, if it hits an obstacle, or when it accelerates or decelerates

quickly.

◊ If the robot is to have jaws for picking up a load, it needs to have a broad

base for stability.

There are more detailed design points in the next two sections.

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Wheels

Road wheelsRoad wheelsRoad wheelsRoad wheels

The main points about a road wheel are its diameter amd the nature of its tread. A larger

diameter is better on a rough or uneven surface because the wheel can more easily ride

up over ridges and is less likely to get stuck in grooves. Also it allows there to be a larger

clearance between the surface and the underside of the chassis.

If the surface is smooth and even, for example the rails of a gantry, small wheels have the

advantage of light weight. It is all too easy for a robot design to finish up by being

heavier than the motor can drive. Using small wheels helps to avoid this.

Tyres help the robot to run without slipping. The simply programmed robots usually

start and stop abruptly. This leads to skidding or slipping. We rely on a robot being able

to run in a straight line but slipping makes it run in irregular curves. Unless it is

continually taking its bearings from a fixed landmark, it soon gets lost. Wheels slip, even

when they have tyres, but tyres help to avoid serious slipping.

Above:Above:Above:Above: Road wheel Road wheel Road wheel Road wheel

and Sports tyre byand Sports tyre byand Sports tyre byand Sports tyre by

Tamiya, seen on theTamiya, seen on theTamiya, seen on theTamiya, seen on the

Scooter.Scooter.Scooter.Scooter.

Below:Below:Below:Below: Road wheels Road wheels Road wheels Road wheels

by Lego in two sizes.by Lego in two sizes.by Lego in two sizes.by Lego in two sizes.

The photo shows theThe photo shows theThe photo shows theThe photo shows the

wheels with andwheels with andwheels with andwheels with and

without tyres.without tyres.without tyres.without tyres.

AboveAboveAboveAbove: Road wheels and: Road wheels and: Road wheels and: Road wheels and

tyres by Meccano.tyres by Meccano.tyres by Meccano.tyres by Meccano.

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A recurring problem with wheels is that the hubs of the selected wheels do not fit on to

the output shaft of the selected motor. There are no standard diameters. Apart from

trying for a different motor or wheels, the solution is to compromise and improvise!

A wheel must be secured to the shaft so that it does not work loose and drop off, and it

does not slip when torque is applied. Often a friction grip between hub and shaft is

adequate, especially for a light-weight robot. If the diameter of the shaft is less than that

of the hub, slip a short length of plastic sleeving, aquarium aerator tubing, or PVC

insulation (stripped from a cable) on to the end of the shaft. Then push the wheel on to

that. Possibly a second layer may be needed to make a tight fit.

If the wheel fits fairly well (so as not to wobble when rotating) slipping can be prevented

by wiring the hub to the shaft (p. 170).

There are several sources of road wheels suitable for robots. Tamiya make a variety of

wheel with tyres, including truck tyres and sports tyres. They also make tanks tracks,

which are sold complete with the wheels to run in the tracks. The wheels and tracks are

boxed as kits and sold by hobby shops.

The constructional sets manufactured by Meccano and Lego include many different types

of road wheel. It is possible to obtain most of these as separate parts from a model shop,

or on the Web (pp. 51-52).

Gear wheelsGear wheelsGear wheelsGear wheels

Gear wheels are often needed for drive transmission and for moving arms and grippers.

They are available as packeted kits of plastic gear wheels of a range of diameters from

various manufacturers. Tamiya produce sets of gears, including motors, that can be

assembled into gearboxes of many different ratios. Meccano and Lego produce gear

wheels too, and the kinds of mechanism that can be built from them are shown in the

photos overleaf.

Gear wheels transmit turning force by engaging their teeth. The number of teeth on the

wheels matters more than their diameters. When talking about gear wheels we speak of

‘24t’ wheels and ‘36t’ wheels, meaning wheels with 24 and 36 teeth.

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When one gear meshes with another the speed (or rate of rotation, or angular velocity) of

one gear relative to the other depends only on the numbers of teeth on each wheel. For

example, wheel A, with 10 teeth, is driven by a motor and engages with wheel B, which

has 40 teeth. As A rotates one revolution its 10 teeth mesh with 10 teeth of wheel B. But B

has 40 teeth, so has turned only a quarter of a revolution. Meshing a wheel with few teeth

against one with more teeth gives a reduction in speed.

In the example, the gear ratio is given by the numbers of teeth on the two wheels. The

ratio is 40 to 10, or 4 to 1, generally written as 4:1. An electric motor turns at high speed,

generally at several thousand revolutions per minute. If a motor turns at, say, 16000 r.p.m

and drives wheel A, then wheel B will turn at a quarter of that speed, which is 4000 r.p.m.

As an equation:

speed of B = speed of A ×

4000 r.p.m. is too fast for turning a robot’s road wheels. The speed would be less if A had

fewer teeth and B had more, but there is a limit on how small we can make A and how

large we can make B. Instead we put a third wheel C on the same shaft as B so that they

turn together. C has few teeth, say 10. Wheel C is meshed with a fourth gear D, with

more teeth, say 40.

The speed ratios are like this:

A to B 4 to 1

B to C 1 to 1 (on same shaft)

C to D 4 to 1

The overall ratio of A’s speed to D’s speed is 4 × 4 = 16 to 1. If the motor and A turn at

16000 r.p.m., D turns at 16000/16 = 1000 r.p.m.

This arrangement of gear wheels is called a gear train. The train described above needs a

few more wheels to give a speed suitable for road wheels, but the principle of meshing

few teeth with more teeth is the same. The simplest way to construct a train of gears is to

build a gearbox from two panels of aluminium sheet or expanded PVC. The panels are

bolted together, usually at their corners.

Opposite are some practical examples of ways of using gears and gear trains.

teeth on A

teeth on B

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The small wheel hasThe small wheel hasThe small wheel hasThe small wheel has

ten teeth and the larger wheelten teeth and the larger wheelten teeth and the larger wheelten teeth and the larger wheel

has 57. The ratio is 5.7:1, so thehas 57. The ratio is 5.7:1, so thehas 57. The ratio is 5.7:1, so thehas 57. The ratio is 5.7:1, so the

larger wheel rotated at aboutlarger wheel rotated at aboutlarger wheel rotated at aboutlarger wheel rotated at about

one sixth of the speed of theone sixth of the speed of theone sixth of the speed of theone sixth of the speed of the

smaller one. Torque is increasedsmaller one. Torque is increasedsmaller one. Torque is increasedsmaller one. Torque is increased

about 6 times,about 6 times,about 6 times,about 6 times,

A worm gear. One rotation of the wormA worm gear. One rotation of the wormA worm gear. One rotation of the wormA worm gear. One rotation of the worm

turn the the 58-toothed wheel by oneturn the the 58-toothed wheel by oneturn the the 58-toothed wheel by oneturn the the 58-toothed wheel by one

tooth. The ratio is 57:1. Torque is hightooth. The ratio is 57:1. Torque is hightooth. The ratio is 57:1. Torque is hightooth. The ratio is 57:1. Torque is high

with this type of gear, Note that this is awith this type of gear, Note that this is awith this type of gear, Note that this is awith this type of gear, Note that this is a

‘one way’ gearing because it is not‘one way’ gearing because it is not‘one way’ gearing because it is not‘one way’ gearing because it is not

possible to rotate the worm by turning thepossible to rotate the worm by turning thepossible to rotate the worm by turning thepossible to rotate the worm by turning the

large gear. The large gear can be turndlarge gear. The large gear can be turndlarge gear. The large gear can be turndlarge gear. The large gear can be turnd

only by rotating the worm, Shafts are atonly by rotating the worm, Shafts are atonly by rotating the worm, Shafts are atonly by rotating the worm, Shafts are at

right angles,right angles,right angles,right angles,

A crown and pinion gear is a A crown and pinion gear is a A crown and pinion gear is a A crown and pinion gear is a

reduction gear with the shafts atreduction gear with the shafts atreduction gear with the shafts atreduction gear with the shafts at

right angles. The smaller gear, theright angles. The smaller gear, theright angles. The smaller gear, theright angles. The smaller gear, the

pinion, has ten teeth. The crownpinion, has ten teeth. The crownpinion, has ten teeth. The crownpinion, has ten teeth. The crown

has 50 teeth. The ratio is 1:5. sohas 50 teeth. The ratio is 1:5. sohas 50 teeth. The ratio is 1:5. sohas 50 teeth. The ratio is 1:5. so

the smaller wheel turns five timesthe smaller wheel turns five timesthe smaller wheel turns five timesthe smaller wheel turns five times

faster than the large wheel.faster than the large wheel.faster than the large wheel.faster than the large wheel.

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A reduction gear train reduces the speed of rotation and it increases the torque. This is

the ‘turning force’, of the shaft. The amount of torque depends on the size of the applied

force and its distance from the centre of rotation. Torque is measured by the sizes of the

force and distance. For example, the torque of the motors used to drive the Quester

(Project 4) is given as 2.1 kgf cm. It is the turning force produced by a force equivalent to

2.1 kg, acting on a wheel of 1 cm radius. A force of half that size (1.05 kgf) acting at

double the distance (2 cm) has the same torque.

The torque produced by force fThe torque produced by force fThe torque produced by force fThe torque produced by force f

acting at a distance d isacting at a distance d isacting at a distance d isacting at a distance d is

T = fdT = fdT = fdT = fd

The motor turns the input shaft which has a pinion A meshing with a crown B. A smallThe motor turns the input shaft which has a pinion A meshing with a crown B. A smallThe motor turns the input shaft which has a pinion A meshing with a crown B. A smallThe motor turns the input shaft which has a pinion A meshing with a crown B. A small

gear wheel C is moulded on to B so they turn as one. C engages with a larger wheel D,gear wheel C is moulded on to B so they turn as one. C engages with a larger wheel D,gear wheel C is moulded on to B so they turn as one. C engages with a larger wheel D,gear wheel C is moulded on to B so they turn as one. C engages with a larger wheel D,

which has a pinion E moulded on it. D/E is a loose fit on the shaft. E meshes with awhich has a pinion E moulded on it. D/E is a loose fit on the shaft. E meshes with awhich has a pinion E moulded on it. D/E is a loose fit on the shaft. E meshes with awhich has a pinion E moulded on it. D/E is a loose fit on the shaft. E meshes with a

larger wheel F which has a pinion G moulded on it. G/H is loose. G meshes with alarger wheel F which has a pinion G moulded on it. G/H is loose. G meshes with alarger wheel F which has a pinion G moulded on it. G/H is loose. G meshes with alarger wheel F which has a pinion G moulded on it. G/H is loose. G meshes with a

larger wheel H and this is a tight fit on the output shaft.larger wheel H and this is a tight fit on the output shaft.larger wheel H and this is a tight fit on the output shaft.larger wheel H and this is a tight fit on the output shaft.

The numbers of teeth are: A = 10, B = 36, C = 14, D = 36, E = 14, F = 36, G = 14, H = 36.The numbers of teeth are: A = 10, B = 36, C = 14, D = 36, E = 14, F = 36, G = 14, H = 36.The numbers of teeth are: A = 10, B = 36, C = 14, D = 36, E = 14, F = 36, G = 14, H = 36.The numbers of teeth are: A = 10, B = 36, C = 14, D = 36, E = 14, F = 36, G = 14, H = 36.

The A to B ratio is 1:3.6, the other three are ratios are all 14:36 or 2.57:1. The overallThe A to B ratio is 1:3.6, the other three are ratios are all 14:36 or 2.57:1. The overallThe A to B ratio is 1:3.6, the other three are ratios are all 14:36 or 2.57:1. The overallThe A to B ratio is 1:3.6, the other three are ratios are all 14:36 or 2.57:1. The overall

ratio is 1:(3.6 × 2.57 × 2.57 × 2.57) = 1:61.ratio is 1:(3.6 × 2.57 × 2.57 × 2.57) = 1:61.ratio is 1:(3.6 × 2.57 × 2.57 × 2.57) = 1:61.ratio is 1:(3.6 × 2.57 × 2.57 × 2.57) = 1:61.

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44

For example, a robot gripper is holding a load of 0.2 kg and the arm of the gripper is 15

cm long. The arm rotates to lift the load. The torque required is 0.2 × 15 = 0.75 kgf cm.

A motor that develops a torque of only 0.5 kgf cm will simplystall when it tries to lift the

load.

The secret of success is a reduction gear train, The Quester’s motors have built-in gearing

with an 82:1 ratio. This reduces the rate of rotation of the output shaft to 1/82 times that

of the motor, giving an output of 70 r.p.m. But, at the same time, it increases the torque to

82 times that of the motor (in practice, a little less because of friction).

The torque quoted for motors depends on operating voltage. The actual torque

developed by a motor is less when it is run on a lower voltage.

Most applications of motors require some form of reduction gearing, but the same result

can sometimes be obtained in other ways. A good example is the steering mechanism

used in the robot toy (p. 249). The belt winds directly around the output shaft. The

diameter of the pulley is 12.5 times that of the shaft, giving a reduction ratio of 1:12.5.

Pulley wheelsPulley wheelsPulley wheelsPulley wheels

A pulley is a wheel with a grove around its rim, known as a race. Pulley wheels are

mainly used for the transfer of force. In the Gantry, for example, they transfer the force of

gravity to the chassis to pull it along the tracks when the winch unwinds. They are also

used in the pulley systen that raises and lowers the hook and some other tools. Another

way in which force is transferred is by a belt drive between two pulleys.

A pair of pulleys of equal diameter simply transfer force over a distance. If their

diameters differ, the result is similar to that of a gear chain. A larger pulley driven by a

smaller pulley rotates at a slower speed but with increased torque.

Pulleys are connected by the drive band, but gears have to be in contact. This makes

pulleys useful for transferring force over a distance. A further advantage of using pulleys

is that drive bands are slightly elastic so the exact diatance between the wheels is not

important. With gears it is essential for the teeth of the two wheels to mesh accurately

together.

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45

Above: Pulleys transfer power over a distance. If theAbove: Pulleys transfer power over a distance. If theAbove: Pulleys transfer power over a distance. If theAbove: Pulleys transfer power over a distance. If the

wheels are of different sizes, they turn at differentwheels are of different sizes, they turn at differentwheels are of different sizes, they turn at differentwheels are of different sizes, they turn at different

speeds. The diameters of the races of these wheels arespeeds. The diameters of the races of these wheels arespeeds. The diameters of the races of these wheels arespeeds. The diameters of the races of these wheels are

36 mm and 12 mm.36 mm and 12 mm.36 mm and 12 mm.36 mm and 12 mm.

36/12 = 3, so one turn of the bigger wheel results in 336/12 = 3, so one turn of the bigger wheel results in 336/12 = 3, so one turn of the bigger wheel results in 336/12 = 3, so one turn of the bigger wheel results in 3

turns of the smaller wheel.turns of the smaller wheel.turns of the smaller wheel.turns of the smaller wheel.

Reverse direction by twisting the drive belt.Reverse direction by twisting the drive belt.Reverse direction by twisting the drive belt.Reverse direction by twisting the drive belt.

The photo shows the ‘rounded triangular’The photo shows the ‘rounded triangular’The photo shows the ‘rounded triangular’The photo shows the ‘rounded triangular’

shape of Meccano hubs and shafts, whichshape of Meccano hubs and shafts, whichshape of Meccano hubs and shafts, whichshape of Meccano hubs and shafts, which

prevents them from slipping, The hubs of theprevents them from slipping, The hubs of theprevents them from slipping, The hubs of theprevents them from slipping, The hubs of the

gear wheels are the same shape.gear wheels are the same shape.gear wheels are the same shape.gear wheels are the same shape.

Shafts can be at right angles and at any otherShafts can be at right angles and at any otherShafts can be at right angles and at any otherShafts can be at right angles and at any other

smaller angle. There is a risk of the belt comingsmaller angle. There is a risk of the belt comingsmaller angle. There is a risk of the belt comingsmaller angle. There is a risk of the belt coming

off if the shafts are too close together.off if the shafts are too close together.off if the shafts are too close together.off if the shafts are too close together.

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46

A disadvantage of pulleys is that the drive can can come off or it may break. Another

problem is that if the load is heavy the band may slip, but this is not necessarily a bad

thing. It introduces elasicity into a system. The jaws of a gripper, for example, close on an

object that is to be securely held. If they grip it loosely, it may fall out. If they grip it too

tightly, they may crush it. The exact point at which the jaws should stop closing on the

object is difficult to determine.

The best approach is to build a little elasticity into the system. One way is to use the

gripper only for elastic objects, such as sponge balls. Another way, which allows the

gripper to handle any kind of object, is to use an elsttic drive band in the mechanim that

closes the gripper. As it closes on the object the band begins to slip. The motor is left

running to maintain the grip, so that the object does not fall out.

Motors

Although motors are electrical and therefore might be a topic for the electronics chapter,

they move the mechanical parts so need to be talked about here. The discussion focuses

on small low-voltage DC motors.

When selecting a motor for a project, one of the main points is its operating voltage, for

this partly decides the size and weight of the battery that must be carried. Motors

running on 12V need eight AA cells. These are too heavy a load for a small robot such as

the Scooter. The Quester is larger and heavier so needs more powerful motors to propel it.

This robot runs on a pair of 12 V motors, powered by a battery of eight AA cells. If we

needed maximum power we would use dry cells, but rechargeable cells are more

economical. Eight LiMH cells produce 9.6 V, and will drive the motor with sufficient

power .

The next point to consider is the gearbox. It is rarely that a robot mechanism can be

drived directly from a motor turning at several thousand of revolutions per minute. If the

motor does not have a built-in gearbox, it is almost certain that you will need to build or

buy one. A built-in gearbox is neater, more convenient, and probably cheaper than a

separate one.

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47

Stepper motorsStepper motorsStepper motorsStepper motors

A typical stepper motor has four sets of coils, arranged so that the rotor is turned from

one position to the next as the coils are energised in a fixed sequence. This is listed in the

table overleaf.

The sequence repeats and at any step, two coils are on and two are off. The sequence of

pulses needed to drive the motor can be provided by a microcontroller.

Finally, there is the matter of coupling the output shaft to the driven mechanism. The

problem that this often raises was mentioned on p. 00. If possible, pick a motor with a

shaft that matches. A 4 mm shaft is usually easier to couple. Special couplers are made for

joining shafts of 4 mm and larger, but these are relatively expensive and bulky. They add

to the robot's weight, which is to be avoided if possible.

A pair of very inexpensive motors runningA pair of very inexpensive motors runningA pair of very inexpensive motors runningA pair of very inexpensive motors running

on 1.5 V to 4.5V. They produce 8 gf cmon 1.5 V to 4.5V. They produce 8 gf cmon 1.5 V to 4.5V. They produce 8 gf cmon 1.5 V to 4.5V. They produce 8 gf cm

and 18 gf cm respectively. These motorsand 18 gf cm respectively. These motorsand 18 gf cm respectively. These motorsand 18 gf cm respectively. These motors

have no gearbox.have no gearbox.have no gearbox.have no gearbox.

A 1.5 V to 3 V motor with an attachedA 1.5 V to 3 V motor with an attachedA 1.5 V to 3 V motor with an attachedA 1.5 V to 3 V motor with an attached

metal gearbox. Low-cost butmetal gearbox. Low-cost butmetal gearbox. Low-cost butmetal gearbox. Low-cost but

comparatively noisy.comparatively noisy.comparatively noisy.comparatively noisy.

A 12 V motor with built-in gearbox. VeryA 12 V motor with built-in gearbox. VeryA 12 V motor with built-in gearbox. VeryA 12 V motor with built-in gearbox. Very

quiet. Output shaft is 4 mm, with a flatquiet. Output shaft is 4 mm, with a flatquiet. Output shaft is 4 mm, with a flatquiet. Output shaft is 4 mm, with a flat

for more secure coupling.for more secure coupling.for more secure coupling.for more secure coupling.

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48

Stepno.

Coil 1 Coil 2 Coil 3 Coil 4

0 On Off On Off

1 Off On On Off

2 Off On Off On

3 On Off Off On

To get from step to step, first coils 1 and 2 change state, then 3 and 4 change state, then 1

and 2, and so on. The result of this is to produce clockwise turning of the rotor, 15° at a

time. If the sequence is run in reverse, the rotor turns anticlockwise. Six times through the

sequence causes the rotor to turn one complete revolution.

The rate at which it runs through the switching sequence depends on the timing of the

program. The motor turns one revolution for every 24 pulses. Varying the pulse rate

varies the speed of the motor. At any step, the rotor can be held in a fixed position by

halting the sequence. If a stepper motor is used for driving something like an arm of a

robot, the arm can be positioned exactly by programming the controller to produce the

required number of pulses. There is no need for linit switches; the robot always knows

where its limbs are. It can move precisely from one position to another simply by

working out how many pulses to generate. Once in the required position, the rotor is, in

effect, locked there and can not move. The torque requird to overcome the magnetic field

holding the rotor in position is several hundred gram-force centimetres.

Another advantage of the stepper motor is that its speed is precisely controllable. It is not

affected by the load on the motor, except perhaps an excessive load, which might

completely prevent the motor from turning. However a stalled stepper motor does not

overheat and possibly burn out its coils, as does an ordinary motor.

The stepper motor may be made to turn 7.5° per step by using a slightly different

switching pattern. Motors with a 1.8° step angle are also produced. When programming a

stepper motor do not feed the pulses to it too fast. At excessive speeds there is the

possibility of dropped steps, which leads to an error in the positioning.

The sequence of pulses for running a stepper motor.The sequence of pulses for running a stepper motor.The sequence of pulses for running a stepper motor.The sequence of pulses for running a stepper motor.

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49

Stepper motors sound ideal for robots but we have not used them in the projects in this

book. One reason is that it is difficult to program pulse production at the same time as

other necessary tasks. Also the motors require large current and at least 12 V operating

voltage.

ServomotorsServomotorsServomotorsServomotors

A servomotor provides feedback of angular position. It has reduction gearing so that the

lever turns relatively slowly (typically taking 0.2 s to turn 60°) when the motor is

running. If required, the angular motion of the lever can be converted to linear or other

forms of motion by connecting it to suitable gearing or lever systems.

The servomotor has a rotary potentiometer on the same spindle as the output lever. The

lever s connected to a mechanism of the robot, for example an arm or the wheels that

steer the robot. If the ends of the potentiometer are connected to 0 V and +V, the output vS

from the wiper of the potentiometer is proportional to the angular position of the lever.

The output from the wiper is sent to an analogue to digital converter in the PIC

controller. The PIC is programmed to know what this voltage should be at any given

stage in the operation. If it is greater or less than the required value, the lever is not in the

correct position.

A servomotor has a built-in potentiometer to feedback information about the lever position to thecontrol circuit.

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50

Solenoids

A solenoid has a many-turned coil of wire, longer than it is wide, and a soft iron core.

When a current is passed through the coil, magnetic forces pull the core into the coil.

Nothing happens when the current is turned off unless there is a spring mechanism (or

gravity) to move the core out of the coil again.

Solenoids are used to provide linear motion, to pull something in a straight line. If the

core is well inside the solenoid to start with, the force on it can be quite large. A 12 V

solenoid can develop a pull of one or two kilograms force. The problem is that the stroke,

the distance that the core moves, is only a few millimetres. This limits the usefulness of

solenoids in robots.

The controller is programmed to switch current to the motor in one direction or the other,

until it has brought the lever to the required angular position. This is what is meant by a

servo control system.

Servomotors are often used in robots and in model aeroplanes and vehicles. Model shops,

especially those specialising in flying model aeroplane kits, usually stock a wide range of

servo motors.

A miniature solenoid suitable forA miniature solenoid suitable forA miniature solenoid suitable forA miniature solenoid suitable for

robotic applications.robotic applications.robotic applications.robotic applications.

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51

The main applications of solenoids are short-stoke actions, such as releasing catch

mechanisms. An electric door latch is operated by a solenoid. They are also built in to

operate electrically controlled valves.

Construction kits

These are a rich source of ready-made parts, such as wheels, angle brackets, girders, and

many more. Construction sets include Meccano, Erector and Lego. Parts are

available as kits or in small quantities of individual types. Even if you intend to make the

parts of ypur robot yourself eventually, an extensive kit of parts is handy for mocking up

a mechanism during the early stages of design.

There are also the more specialist kits marketed by companies such as Tamiya. Their road

wheels and pulleys are used in several of the projects in this book The local model or

hobby shop is a good source of these.

Suppliers

Local stores are the first place to try, particularly:

• Model shops — for aluminium, brass, plastic and wooden stock in small dimen-

sions. Also constructional sets and kits of parts such as wheels and tyres and items

such as servomotors.

• DIY stores — for aluminium and wooden stock in the larger sizes, tools, and

specialist items in departments such as plumbing and gardening, which can be put

to use in unconvential ways for building robots.

• Electronics stores — for motors, solenoids, nuts and bolts in small sizes.

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52

• Plastics retailers — for expanded PVC sheet and other materials. Look in the

Yellow Pages.

New suppliers of robotic components are continually appearing and no list can be up to

date. Try running a browser, such as Google, or Yahoo! Type in a general keyword, such as

‘robot’, ‘robotics’, ‘Meccano’, ‘Tamiya’, ‘Lego’, ‘motors', or ‘wheels’. There are lots more

you can think of.

This approach hardly ever fails to produce the address of an on-line supplier or two that

you had not heard of before.

Apart from advertisements in hobby magazines, the World Wide Web is the best point of

contact for mail order supplies. Here are a few addresses:

www.robotstore.com — a very wide range of parts by mail order over the web.

www.legoshop.com — for Lego parts and sets by mail order over the web.

www.tamiya.com — on-line descriptions of products and a list of agents world-

wide.

www.brickline.com — Lego parts.

www.robotbooks.com — many interesting titles.

www.budgetrobotics.com.

www.robohoo.com

www.hobbyengineering.com — lots of robotics parts, components, and kits.

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